One known method for manufacturing a tuning fork vibrator for an angular velocity sensor by dry etching is shown in FIG. 7. FIG. 7 shows plasma source 100 for dry etching, travel direction 101 of plasma emitted from plasma source 100, resist film 102 as a dry-etching mask and wafer 103.
Resist film 102 is provided with openings to form a plurality of tuning fork vibrators in wafer 103. Resist film 102 is first applied on a main surface of wafer 103 and is then dry etched with plasma emitted from plasma source 100 so as to manufacture tuning fork vibrators. Travel direction 101 of the plasma shown in FIG. 7 is not at an equal angle with respect to the entire surface of wafer 103. More specifically, the plasma is applied in the direction of the normal (perpendicular) to the main surface of wafer 103 just below and near plasma source 100, but is applied at a smaller angle of emission as the region on the main surface gets farther from plasma source 100.
FIGS. 8A-8G show processes of a method for manufacturing a tuning fork vibrator for an angular velocity sensor, and are enlarged views of the area inside circle “P” shown in FIG. 7. FIG. 8A shows that wafer 103 made of a silicon substrate is provided on a main surface thereof with resist film 102a, resist film 102b and resist film 102c. Resist films 102a and 102b have opening 104a therebetween, and resist films 102b and 102c have opening 104b therebetween. Openings 104a and 104b are formed by emitting plasma from plasma source 100 in travel direction 101 as shown in FIG. 7 on the main surface of wafer 103 using resist films 102a, 102b and 102c as masks. Emission direction 101 of the plasma is tilted from the normal to wafer 103. In other words, emission direction 101 of the plasma is not orthogonal (at 90 degrees) to the main surface of wafer 103. As a result, side surface 106 and side surface 109 of openings 104a and 104b, respectively, are applied with and etched by the plasma, thus becoming tilted surfaces, not vertical surfaces.
Opening 104a has side surface 106, side surface 108 and bottom 107. Opening 104b has side surface 109, side surface 111 and bottom 110.
FIG. 8B shows that openings 104a and 104b formed in the process of FIG. 8A are coated with protective film 105. Protective film 105 is formed to minimize the influence of the side etching.
In FIGS. 8C and 8E, on the other hand, side surfaces 108 and 111 opposed to these side surfaces are hardly influenced by the plasma emission because of being in the shadow of resist films 102b and 102c, respectively. In other words, these side surfaces are not influenced by the side etching and are left in parallel with the normal direction of the main surface of wafer 103.
FIGS. 8C-8F show the repetition of the processes of FIGS. 8A and 8B. More specifically, FIGS. 8C and 8F show openings which are different in depth from those in FIG. 8A, but are nearly the same in shape as them. FIGS. 8D and 8F show openings which are different in depth from those in FIG. 8B, but are nearly the same in shape as them. Protective film 112 shown in FIG. 8D is formed for the same purpose as protective film 105 shown in FIG. 8B. That is, to minimize the influence of the side etching on side surfaces 106, 108, 109 and 111 shown in FIG. 8C.
FIG. 8G shows that arms 120, 121 and 122 of tuning fork vibrators are separated from wafer 103 by a final dry etching with plasma applied on protective film 113 shown in FIG. 8F. Protective film 113 shown in FIG. 8F is formed for the same purpose as protective films 105 and 112.
In FIG. 8G, side surfaces 114 and 116 are tilted in travel direction 101 of the plasma as the result of the plasma side etching, in the same manner as surfaces 106 and 109 shown in FIG. 8A that correspond to the side surfaces of arms. Side surfaces 115 and 117, which are hardly influenced by the plasma side etching, are left nearly in parallel with the normal to wafer 103.
FIGS. 9A, 9B and 10 show the state of displacement of a tuning fork vibrator when driven in the X-axis direction, the tuning fork vibrator being manufactured by the method for manufacturing a tuning fork vibrator for an angular velocity sensor shown in FIGS. 7 and 8. FIG. 9A is a plan view of the tuning fork vibrator driven in the X-axis direction; FIG. 9B is a side view of FIG. 9A; and FIG. 10 is a cross sectional view taken along the line C-C of FIG. 9A.
In FIGS. 9A and 9B, arm 120 and arm 121 are connected to each other and supported by base 130. Arms 120 and 121 have main surface 135 and main surface 136, respectively. Arm 120 is provided on main surface 135 with drive unit 140 and drive unit 141. Arm 121 is provided on main surface 136 with drive unit 142 and drive unit 143. Arm 120 is further provided with detection unit 150 on main surface 135, and Arm 121 is further provided with detection unit 151 on main surface 136.
FIG. 10 shows arm 120 on its left side, and arm 121 on its right side. Arm 120 is provided, on outside 120as of main surface 135, with bottom electrode 140a, piezoelectric film 140b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 140c, which are stacked in this order.
Arm 120 is further provided, on inside 120au of main surface 135, with bottom electrode 141a, piezoelectric film 141b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 141c, which are stacked in this order.
Arm 120 is further provided, on approximately center 160 of main surface 135, with bottom electrode 150a, piezoelectric film 150b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 150c, which are stacked in this order. Electrodes 150a, 150c and piezoelectric film 150b are made nearly symmetric with respect to center 160.
Drive unit 140 shown in FIG. 9 is made up of bottom electrode 140a, piezoelectric film 140b and top electrode 140c shown in FIG. 10. Drive unit 141 is made up of bottom electrode 141a, piezoelectric film 141b and top electrode 141c. Detection unit 150 is made up of bottom electrode 150a, piezoelectric film 150b and top electrode 150c. 
On the other hand, arm 121 on the right side of FIG. 10 has a structure similar to arm 120 described above. More specifically, arm 121 is provided, on outside 121as of main surface 136, with bottom electrode 143a, piezoelectric film 143b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 143c, which are stacked in this order.
Arm 121 is further provided, on inside 121au of main surface 136, with bottom electrode 142a, piezoelectric film 142b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 142c, which are stacked in this order.
Arm 121 is further provided, on approximately center 161 of main surface 136, with bottom electrode 151a, piezoelectric film 151b subjected to a polarization treatment in the direction perpendicular to its film surface and top electrode 151c, which are stacked in this order. Electrodes 151a, 151c and piezoelectric film 151b are made nearly symmetric with respect to center 161.
Drive unit 142 shown in FIG. 9 is made up of bottom electrode 142a, piezoelectric film 142b and top electrode 142c shown in FIG. 10. Drive unit 143 is made up of bottom electrode 143a, piezoelectric film 143b and top electrode 143c. Detection unit 151 is made up of bottom electrode 151a, piezoelectric film 151b and top electrode 151c. 
The following is a description about driving the tuning fork vibrator in the X-axis direction.
In FIG. 10, top electrodes 140c and 143c are applied with drive voltages of the same phase. This shrinks piezoelectric films 140b and 143b formed on the respective top electrodes in the Y-axis direction. On the other hand, top electrodes 141c and 142c are applied with drive voltages having a phase opposite to the drive voltages to be applied on top electrodes 140c and 143c so as to stretch piezoelectric films 141b and 142b in the Y-axis direction. As a result, as shown in FIG. 9A, arms 120 and 121 vibrate outwards from each other in the X-axis direction.
The application of the dry etching causes side surfaces 114 and 116 of arms 120 and 121, respectively, to be tilted in travel direction 101 of the plasma as shown in FIGS. 7, 8 and 10. The tiltings of side surfaces 114 and 116 cause vibration which vibrates arms 120 and 121 in the X-axis direction, and at the same time, outwards from each other in the Z-axis direction.
As a result, piezoelectric film 150b as a component of the detection unit is applied with the stress to stretch it in the Z-axis direction. Piezoelectric film 151b as a component of the other detection unit is applied with the stress to shrink it in the Z-axis direction. These stresses cause top electrodes 150c and 151c as components of the respective detection units to have charges with polarities opposite to each other. Thus, just driving the tuning fork vibrator in the X-axis direction causes top electrodes 150c and 151c of the detection units to have electric charges (unnecessary signals) which make it seem as if an angular velocity were applied around the Y axis, although it is not applied.
In the aforementioned method for manufacturing a tuning fork vibrator for an angular velocity sensor, the arms of the tuning fork vibrators formed in wafer 103 gradually change in cross section from rectangular to trapezoidal from the center of wafer 103 to the periphery. More precisely, the tuning fork vibrators have arms different in cross section depending on the position in wafer 103 at which the tuning fork vibrators are formed. As a result, when vibrating in the X-axis direction, the tuning fork vibrators formed far from the center of wafer 103 inevitably cause unnecessary vibration components in a direction (the Z-axis direction) other than the direction in which to vibrate the tuning fork vibrators.
FIG. 11 shows the generation amount of signals which are unnecessary to the angular velocity sensor and are generated on the sensing electrodes when the tuning fork vibrators formed at different positions in wafer 103 are made to vibrate in the X-axis direction. The horizontal axis shows the distance from the center of wafer 103 in the X-axis direction, that is, the position in the X-axis direction. The vertical axis shows the size of the unnecessary signals generated on the sensing electrodes, the size being expressed in an arbitrary unit.
The generation of such unnecessary vibration components can be controlled by adopting, for example, an adjustment method disclosed in Japanese Patent Unexamined Publication No. 10-132573. In this adjustment method, each tuning fork vibrator formed in wafer 103 is provided with a mask (unillustrated) having openings in such a manner that the mask is pasted integrally on each tuning fork vibrator. In this state, the arms of each tuning fork vibrator are continuously weighed to increase or decrease the weight until no unnecessary vibration components are generated in a direction other than the direction in which to make the tuning fork vibrate.
However, in the above-described conventional angular velocity sensor and method for manufacturing it, the tuning fork vibrators in wafer 103 have arms different from each other in cross section depending on the positions in wafer 103 at which the tuning fork vibrators are formed. Therefore, if the adjustment method is adopted, each tuning fork vibrator must be covered with a mask having openings in such a manner that the mask is pasted integrally on the tuning fork vibrator so as to adjust the shape in cross section of its arms, making it inevitable for the finished angular velocity sensors to have a large thickness. As another inconvenience, the shape in cross section of the arms must be adjusted for each and every angular velocity sensor to complete all the angular velocity sensors.
Therefore, the present invention has an object of providing an angular velocity sensor capable of controlling the generation of signals which are unnecessary to the angular velocity sensor and are generated in the sensing electrodes when the tuning fork is made to vibrate in the X-axis direction, the angular velocity sensor also being thin in thickness and requiring no individual adjustment. The present invention has another object of providing a method for manufacturing such an angular velocity sensor.